Neutronic reactor control



Oct. 14, 1958 H. E. METCALF NEUTRONIC REACTOR CONTROL Filed June 4, 19457 Sheets-Sheet 1 Flfil- Oct. 14, 1958 H. E. METCALF NEUTRONIC REACTORCONTROL Filed Jun 4, 1945 '7 Sheets-Sheet 2 FIE- wzgw

Oct. 14, 1958 H. E. METCALF NEUTRONIC REACTOR-CONTROL Filgd June 4, 19457 Sheets-Sheet 3 FIE- 5 Oct. 14, 1958 H. E. METCALF 2,856,336

umumouxc REACTOR CONTROL Filed June 4, 1945 7 Sheets-Sheet 4 Oct. 14,1958 H. .E. METCALF 2,856,336

NEUTRONIC REACTOR CONTROL Filed June 4, 1945 7 Sheets-Sheet 5 Oct. 14,1958 H. E. METCALF 2,

NEUTRONIC REACTOR CONTROL Filed Jupe 4, 1945 7 Sheets-Sheet 6 w A L Q Q[L Q g N N 8 Oct. 14, 1958 H. E. METCALF 2,856,336

NEUTRONIC REACTOR CONTROL Filed June 4, 1945 I 7 Sheets-Sheet 7 underhigh neutron densities.

' bardment of additional U atoms.

United States Patent NEUTRONIC REACTOR CONTROL Herbert E. Metcalf,Chicago, Ill., assignor to the United States of America as representedby the United States Atomic Energy Commission Application June 4, 1945,Serial No. 597,474 4 Claims. 01. 204 1s4.2

My invention relates to the general subject of nuclear fission and moreparticularly to means and methods for developing and controlling aneutronic reaction by nuclear fission of uranium or other fissionablematerial.

It is known that the isotope U in natural uranium can be split orfissioned by bombardment with thermal neutrons, that is, .neutrons inthermal equilibrium with the surrounding medium, to provide undercertain conditions a self-sustaining neutronic reaction operating Thefission neutrons produced by nuclear fission give rise to new fissionneutrons in sufliciently large numbers to overcome the neutron losses inthe system. Since the result of the fission of the uranium nucleus isthe production of two lighter elements with great kinetic energy, plusapproximately a statistical average of two neutrons for each fissionalong with beta and gamma radiation, new elements, as well as a largeamount of power in the form of heat, can be made available in aself-sustaining system.

Most of the neutrons arising from the fission process are set free withthe very high energy of above one million electron volts average and aretherefore not in condition to be utilized efficiently to create newneutrons by thermal fission in U The fast neutrons from fission, afterthey are created, must be slowed down to thermal energies by use of aneutron moderator before they are most effective to produce freshfission by bom- It is necessary that the neutrons be slowed down withoutmuch absorption until they reach thermal energies and then mostly enterinto the uranium rather than into any other element to provide aself-sustaining nuclear chain reaction. Deuterium in the form of heavywater, carbon in the form of graphite, or beryllium in the form ofeither oxide or metal may be used as a moderator or slowing mediumwithout too great a loss of neutrons by capture thereof by the atoms ofthe moderator.

The fissionable material is distributed in the moderator in such amanner that the neutrons, when slowed to thermal energies, are in aposition to enter the uranium or other fissionable material to producenew fission thereof. The combination of the fissionable material and themoderator is referred to as a neutronic reactor and the distribution ofthe fissionable material in themoderator. is referred to as the geometryof the reactor. The fissionable material may be distributed throughoutthe moderator in a regular pattern and a reactor incorporatln g a solidmoderator, such as graphite or beryllium, may be referred was a lattice.7

The ability of any reactor or lattice to remain selfsustaining by therelease of neutrons from the uranium depends upon the number of new fastneutrons released by fission with respect to the original fast neutronsin the system. Thus, the ratio of the';number of fast neutrons producedby the fissions in a generation to the original'number' of fast neutronsin a'reactor of infinite size using specific materials may be called thereproduction or multiplication factor of the reactor and is p 2,856,336Patented Oct. 14, 1958 ice denoted by the symbol K. By making Ksufiiciently' greater than unity to create a net gain in neutrons and bymaking the reactor suificiently large that this gain is not entirelylost by leakage from the exterior surface of the reactor, aself-sustaining neutronic reacting system can be built to produce newelements and fission products as well as power in the form of heat bynuclear fission of natural uranium by thermal neutrons. The heatisdissipated from the reactor either by natural'radiation or by coolingmeans such as by circulation of a coolant through or around the reactor.The neutron reproduction ratio denoted by the symbol R (sometimes knownas efiective K) in a system of finite size difiers from the Y factor Kby the external leakage factor, and must be.

sufliciently greater than unity to permit the neutrondensity to riseexponentially in the reactor. continue indefinitely if not controlled ata desired neutron density corresponding to a desired power output.

It is a principal object of my invention toprovide a method andapparatus for controlling the speed of rise 7 of the reaction in aneutronic reacting system and to limit the reaction to a safe levelwithin the heat dissipating capabilities of the system.

In constructing a neutronic reactor, the fissionable mathe naturalneutron losses and the neutronic reaction be-' comes self-sustaining,and the neutron density continues to rise exponentially. The continuanceof such a condition would be very dangerous because the density oftheneutronic reaction increases exponentially with time and would soonexceed that corresponding to the heat dissipating capacity of thereactor.

It is a further object of my invention 'to provide a method ofconstructing a neutronic reactor and means preventing a nuclear reactionduring the construction of the reactor. It is also an object of myinvention to control the reaction effectively after construction of thereactor to an overall size exceeding that necessary to maintain aself-sustaining neutronic reaction. v

During the neutronic reaction fission products including such elementsas samarium are formed from the fissionable uranium isotope U Samarium,as well as some other elements formed in this manner, has very highneutron absorption characteristics which may increase the total neutronabsorption of the reactor with consequent decrease in the reproductionfactor. However, the absorption of neutrons by the uranium isotope Uleads to the production of 94 a new element often referred to asplutonium, symbol Pu. able to an even greater degree than the isotope Uso that formation of plutonium tends to increase the reproduction factornotwithstanding decrease in the U 5 content of the uranium in thereactor. The operating time of a neutronic reactor of the type describedis practically unlimited being measured in terms of hundreds of yearsand the present lapse of time has. not been sufiicient to determineaccurately whether a decrease or increasein the reproduction factor willbe the ultimate result. Thus, the end result after a long period of timemay be either an increase or a decrease in the available K forsustaining the neutronic reaction. Consequently, it is desirable tobuild the reactor initially in such a manner as to allow futurecompensation for either an increase or decreasein neutronic activitywith continued operation. This is Such rise will The plutonium isfission- I particularly true inasmuch as it would be very diflicult toreconstruct'a neutronic reactor utilizing a solid moderator afteroperation was once initiated because of the radioactive nature. of thefission products formed by the reaction; For example, afteroperation athigh neutron den sity, the radioactivity of the materialsin the reactormay be exceedingly high so th-atit would be impractical for operatingpersonnel to add or remove one or more layers from the structure for thepurpose of overcoming .increased ordecreasedneutron losses through theformation of products causing variation in the reproduction factor.

Itis thus-a further object of my invention to provide a reactor whichmay be initially constructed in such a mannet to allow for. either anincrease or decrease in neut ronic. activity with time, Whilesimultaneously maintainmg adequate control overthe neutronic reactionwhereby the. reaction maybe maintained at safe levels.

During the interchange, of neutrons in a reactor comprising bodies ofuranium of any size in a slowing medium, neutrons may be lost in fourways: by absorption. in the uraniummetal or compound, by absorption inthe slowing-niaterialormoderator, by absorption in impurities present inthe'reactor constituents, and by leakage from the. reactor. ordermentioned.

Natural uranium, particularly by reason of its U content, hasanespecially strong absorbing power for neutrons when they :have beenslowed down to a moderate energy of approximatelyfive electron volts,this amount of energy. being .termed a resonance energy. Other, but notsoTsignificant, resonance energies also cause or result in absorption.The absorption in uranium at these energiesds termed the uraniumresonance absorption or capture. It is caused. by the isotope U and doesnot result-in fission but leads to the creation of the relativelystable. nucleus.94 referred to hereinafter as. plutonium, symbol. Pu. Itis not to be confused with absorption or captureofneutrons byimupurities, referred to later. Neutronresonance absorption in uraniu'm'may take place either onlthe surface of the uranium bodies, in whichcaseithe. absorption is known as surface resonancesabsorptiomor itmaytake place further in the interior of the uranium body, in which casethe, absorption is known as volume-resonance absorption. Volumeresonance absorption is due to the fact that some neutrons makecollisionsiinside' the uranium body and may. thus arrive at resonanceenergies -therein. After successfully reaching. thermal velocities,about 40 percent .of the neutronsare als'o'subject to capture by U?without fission, leading to the p'rb'ductio'ntof 94 Irrespective ofWhether the neutron resonance. absorption inU3 is on the surface or inthe volume of theuranium body; 94 is produced by the resonanceabsorption according-to thefollowing process:

These losses will be considered in the :U -Ffl 1 MU [6 ml e. v. of raysnot necessary all of one frequeney.]

23minutes nNp +811 m; e. v. 6-, no 'y rays] 2.3 days na D luPu+B'[600,m. e. v. upper fienergy limit. Also 2 yrays, 400 convertedto'electrons] It is' possible by proper physical arrangement of. thematerials to'reducei substantially uranium resonance absorption; 1 Bythe use of light elements-as described above for slowing materials,"fewer collisions are required to slow theneutrons to thermal energieswith large increments :of "energy loss between} collisions, thus.decreasing thepr'obability ofa neutro'nfbeing at a resonance energy asit'entersauranium atom. Dhringthe slowing process, however,- neutronsarediffusing through the moderator over random paths and distances so'thatthe uranium is notonly exposed to thermal'neutrons but also to neutronsof energies varying between the energy of fission and v.- and 270 kv.about; 6

t '41. thermal energy. Neutrons at uranium resonance energies will, ifthey enter uranium at these energies, be absorbed on the surface of auranium body whatever its size, giving rise to surface absorption. Anysubstantial reduction of overall surface of the same amount of uraniumwill reduce surface absorption, and any such reduction in surfaceabsorption will release neutrons to enter directly into the chainreaction. 1'

For a given ratio of slowing material to uranium, surface resonanceabsorption losses of neutrons in the uranium can be substantiallyreduced by a large factor from the losses occurring in a mixture of fineuranium particles and a slowing medium when the. uraniurrris aggregatedinto substantial masses in which the mean spatial diameter is at leastlcentimeter for natural uranium metal and when the mean spatial diameterof the bodies is at least 2.4 centimeters for natural uranium oxide (U0in graphite. An important. gainris thus made in the number of neutronsmadedirectly available for the. chainreaction. A similargainismadewhenrthe uranium has more than. the natural contentof fissionable.material such as when. enriched with plutonium. Consequently, theuraniumis placed in the reactor in the form of spaced uranium masses orbodies ofsubstantial size, preferably either of metaLoXide, carbide,or.combinations. thereof. The uranium bodies can be inthe form: oflayers, rods, or cylinders, cubes or-spheres, or. approximate shapes;dis-. persed throughout the graphite, preferably insome geow metricpattern. Optimumconditions are obtained with natural uranium byusingmetalfspheres.

Following initiation of the reaction, the. increased loss of neutrons,such' as by the formation. of fission products highly absorbent toneutrons, may be compensated, .by. varying the volume to surface ratioofjthe uranium bod-- ies assuming a non-optimum. volume to surfaceratiowas initially used in the construction. Thus, the reactor may be builtto substantially, criticalsizeandthe reaction con.- trolled by variationof the volume to surface ratio. However, the uranium bodies may; beinaccessible for removal and replacement in a new. geometry. Inaddition, such a control lacks precision andliis cumbersome, and erraticin operation. Such variation ofthis ratiofjs I also difiicult to providein practice because of the high"radio activities. of the fissionproducts inthe uranium bodies.

It is thus a further object of my invention to. provide. a method andmeans for controlling a neutronic reactor that is positive, precise,andreliable in action and that may be efiected Without exposure of.personnel to high radio activities.

Thethermal neutrons, are also subject to capture by the moderator.,VVhile carbon and-berylliumhave veryxsmall absorption characteristicsfor thermal neutrons,.an appreciable fraction of thermal neutrons(about'l0 percent of the neutrons present in the reactor under usualconditions with graphite) is lost by. capturein theslowing' materialduring diffusion therethrough. It'is therefore desirable to have theneutrons reach. thermal energy promptly and promptly enter. the uranium.

In addition to the above-mentionedlosses, whichareinherently a part ofthe neutronic reaction-process; impurities present in both the.moderator and theuraniumadda very important neutron lossfactor in thechain The efiectiveness of variouselements as neutronabsorbers variestremendously. Certain elements 'suchas'boron cadmium, samarium,gadolinium, and'some others, if present even in a fewparts' per million,could very "likely prevent a self-sustaining .chain reactionfrom takingplace. It is, highly important, therefore, torrem'ove asfar; as possibleall impurities capturing neutronsto the detriment of the chainreactionfrom both theqslowing material and the uranium. If theseimpurities are presentintoo great quantity, the. self-sustainingneutronic reaction cannot be attained. The permissible amounts ofimpurities: will vary for each specific geometry, depending upon suchused-that is, whether natural or enriched, whether as metal or as oxide.The type of moderator used-for example, whether graphite orberyllium-also influences the effect of impurities, as do the weightratios between the uranium and the moderator. Elements such as oxy-v genmay be present and, as previously suggested, the uranium may be in theform of oxide, such as U or U 0 or carbide, but the metal is preferred.Nitrogen may be present in fairly large amounts, and its effect on thechain reaction is such that the neutron reproduction ratio of the systemmay be changed by changes in atmospheric pressure. The effect may beeliminated by enclosing or evacuating the reactor if desired.

The effect of impurities on the optimum reproduction factor K may beevaluated conveniently by means of certain constants known as dangercoefficients which are assigned to the various elements. These dangercoefiicients for the impurities are each multiplied by the percent byweight of the uranium in the reactor of each corresponding impurity, andthe total sum of these products gives a value known as the total dangersum. This total danger sum is subtracted from the reproduction factor Kas calculated for pure materials and for the specific geometry underconsideration.

The danger coefficients are defined in terms of the ratio of the weightof impurity per unit mass of uranium and are based on the cross sectionfor absorption of It is therefore a further object of my invention topro vide a neutronic reactor into which relatively large quantities offoreign materials and substances may be introduced without the danger ofthe introduced materialsrendering the reactor inoperative. It is a stillfurther object to provide a reactor wherein the neutronic reaction maybe initiated in the presence of relatively large quantities of foreignmaterials introduced within the reactor without sacrifice of controlover the neutronic reaction.

It has been proposed to utilize the high absorbing action of someelements as a variable neutron absorber to control the reaction inaccordance with the number of thermal neutrons of the various elements.These values may be obtained from physics textbooks on the subject andthe danger coeflicient computed by the formula wherein 0' represents thecross section for the impurity and 0- the cross section for the uranium,A the atomic weight of the impurity and A the atomic weight for uranium.If the impurities are in the carbon or beryllium moderator, they arecomputed as their ratio of the weight of the uranium of the system.

As a specific example, if the materials of the system underconsideration have .0001 by weight of H, Co, and Ag, and the dangercoefiicients for these elements are 12, 13, and 17, respectively,-thetotal danger sum in K units for such an analysis would be:

.0001X12+.0O0l 13 +.000l X 17=.0042 reduction in K This would be arather unimportant reduction in the reproduction factor K unless thereproduction factor for a given system, without considering impurities,is very nearly unity. If, on the other hand, the impurities in theuranum are similar amounts of Li, Co, and Rh, having danger coeflicientsof 320, 13, and 42, respectively, the total danger sum would be:

.0320+.00l3+.0042=.03 75 reduction in K This latter reduction in thereproduction factor K for a .given reactor would be very serious .andmightwellreduce the reproduction factor below unity for certaingeometries so as to make it impossible to eflect a selfsustainingneutronic reaction with natural. uranium and graphite, but might stillbe permissible when using enriched uranium in a system having a high Kfactor. The examples given will also illustrate how small amounts ofimpurities, if built up in the system from fission elements,

can change the value of K and thus change the operating conditions ofthe reactor.

It is often desirable to subject various materials to high neutrondensities for investigation of their neutron.

neutrons absorbed. By introducing neutron absorbing elements in the formof rods or sheets into the interior of the reactor, preferably in themoderator between the separated uranium masses, the neutronicreproduction ratio of the reactor can be changed in accordance with theamount of the absorbing material exposed to the neutrons in the reactor.While such a control is effective in limiting the reaction within safeneutron densities, adequate provision is not made for changes inreproduction factor of a solid moderator reactor with prolongedoperation, the control being dependent entirely upon the number ofneutrons absorbed from the reaction.

Thus, it is a further object of my invention to provide a solidmoderatorreactor wherein the control is substantially. independent of the numberof neutrons absorbed by the reactor controls, proportionate neutronabsorption being substantially constant for all degrees of reactorcontrol and almost exactly constant for small values of reactor control.

When the uranium and the moderator are of such purity and the uranium isso aggregated that fewer neutrons are parasitically absorbed than aregained by fission, the uranium will support a neutronic chain reactionand thereby provide an exponential rise in neutron density if theoverall size of the reactor is sufliciently large to overcome the lossof neutrons escaping from the reactor. Thus, the overall size of thereactor is important.

The overall size of the reactor will vary, depending upon the K factor.If the reproduction factor K is greater than unity, the number ofneutrons present'will increase exponentially and indefinitely, providedthe reactor structure is made sufficiently large. If, on the contrary,the structure is small, with a large surface-tovolume ratio, there willbe a rate of loss of neutrons from the structure by leakage through theouter surfaces, which may overbalance the rate of neutron productioninside the structure so that a chain reaction will not beself-maintaining. For each value for the reproduction factor K greaterthan unity, there is thus a minimum overall size of the reactor known asthe critical size, above which the rate of loss of neutrons by diffusionto the walls of the structure and leakage away from the structure isless than the rate of production of neutrons within the reactor, thusmaking the neutronic chain reaction self-sustaining. The rate ofdiffusion of neutrons away from a large reactor, in which they are beingcreated, through the exterior surface thereof may be treated bymathematical analysis when the valueof K is known, as the ratio of theexterior surface to the volume becomes less as the reactor is enlarged.

In the case of a spherical structure employing uranium bodies of anyshape or size imbedded in a moderator such as graphite, the followingformula gives the critical overall radius:

Critical sphere R ft., Kl=7.4/R

For a parallelepiped structure rather than spherical, the critical sizecan be computed from theformula Z here a, b, and c are th e lengths ofthe sides infect. I

ssesses Thecrit-ical-size for-a cylindricaL structure irrespective ofthe shape of the uraniu m--bodies, is given jbythe;

formular I Cylinder height h ft.

Radius R ft.

The above formulae determine the criticalqsize. of the; reactor and thissize must be exceeded slightly to initiator:

the neutronioreaction. Consequently it has, been pro posedgtoinsertneutrom abs.orbing; material in. the ,form of d n ts eir -acto rin;cqn tructiontheter of audio partially removesthe absorbing material;53ft? reaction.

able neutron absorption control, that may .be exce edinglydangerousinasmuch as asmall@decrease in;neutromab:

sorptionr may cause such a rapid BX1JQI1I1iifliwLTiSS3 in neutrondensity that it is impossible ,to maintain therise within safe limits.

Itv is those further object 1 of.inn-invention;to proyide .a neutronicreactor of a size. considerably-.greater--than:

critical size to. allow. for variation .in the ;.reproduction ratiothereof during prolougcd Op ration that may be controlled with minimumdanger of exceeding the permissible neutron density. I

The above objects and..other objects, features, and

advantages of my invention .will become apparent ,upon a considerationof the following description of a system embodying my invention, whenread. in. view. of the accompanying drawings, which 'illus'trate a.neutronic reacting system wherein my invention may be utilized.

In the drawings:

Fig. l is a diagrammatic, perspective view of a neutronic reactorcompletely enclosed in a concrete, shield;

Fig. 2 is a plan view in cross-section. ofthe structure shown in Fig.1taken alongthe line 2.'2;

Fig. 3 is an enlarged fragmentary detaiLsectional view through a portionof the graphite and uranium structure in a neutronic reactor;

Fig. 4 is a plan view partially in cross-section of the enclosed reactortaken along the line 44 of. Fig. 1;

Fig. 5 is cross-sectional view of the structure in Fig. 1 taken on theline 5-5 of Fig. 4; i

Fig. 6 isan enlarged fragmentary cross-sectional view partially inelevation of a portion of thestructure shown in Fig. 5 taken along theline 6 6 thereof showing the positioning of the control platesin thereactor;

Fig. 7 is an'elevation view ingress-section of a modified form ofreactor incorporating my invention;

Fig. 8 isan enlarged elevation view of the modified reactor shown inFig. 7 taken. alongthe line 8- 8 thereof;

Figs. 9a11d'1O'are representative curves showing variaw tion of neutrondensity within a reactorl for two diiferent positions of thecontrol.plates; and

Fig. ll'is aschematic wiring diagram showing a circuit for controlling aneutronic reactor in accordance with my invention.

It willbe understood that such criteria forthe production of a neutronicchain reaction as the .exact nature and purity of the materialsemployed,,dimensions, spaoings, cooling systems and other specificationsfor making a neutronic reactor self-sustaining, are not theinvention ofthe present inventor, such criteria for operativenessbeing well known intheart, as exemplified by the cop ending application of Fermi andSzilard, Serial No. 56 85904, filed December 19, 1944, now Patent No.2,708,656, among others.

Asa self-sustaining chain reaction in ;a solid moderator such astcarbonor beryllium is not possible,with natural uranium withoutaggregation of the uranium, I prefer to illustrate my invention bydescniption of a simple, conductively cooled self-sustaining neutronicreacting system in which the reactor portion .is parallelepiped in- 5materials.

, shape, and in *WhlQlIflfifllQllI'Ztl metal bodies, bedded inhighjqualitygraphite; areutilized, The efiect of *impuritiesjn-thecarbbn is substantially the same as in'the illranium since the neutronsdilfusetr'eely in both Referring first toFigs. 1 and 2 illustrating thecompleted structure, side=.-walls 10"areerected on a heavy foundationllnbotlppreferably of pouredtsonorete about 5 feet thickg leavinga-vault space 14 insido walls10 -in=whichy A is built up,.as will -beexplained later, a neutron-lo reactor 15 (Fig. 2} surrounded .by agraphite reflectorlfiu The it reflector is built withxa graphiteprojection 11 that e t- 7 tends to a plane fiush:withwthe outer surfaceof front wall 118Li'and .with anoppositely disposed graphiteproj'cctionpllllrthabextends .to. a plane flushbwith the: outer surfaceof rear wall 10.x. The walls are also: 5 feet-thick, formed ofconcretebricksl2l Thetop-of the/structure is closed withcovering. bricks 22,also of concrete and.

5 feetthick;

The side jwalls .2100. are r pierced .to provider a horizontal. itchannel 23rextending to the. centenzofthe reactor lfnncar the. centralportiontthereof. T he. channel 23 maybexluscd to introducevariousmaterials within .the. reactor, and as a channel through whichbothsfast and slow neutronsiand .25 gamma rays. may escape: to the.outside ofithesreactor for use in irradiationntestsgand studies: made.outside lthe' r,

' reactor-shield. When not in use the channeln23mmaym be plugged with agraphite rod; (not shown) extendingrto. the center of the reactor. t 30Within 5 the reflector 16 and closely: adjacent; the 1'67:

actor 15 is positioned an ionization chamber 24, connected to theexterior by wire line 25 lying within a shield conduit 26. Theionization chamber is useful for determining neutron. density duringoperation, as described later in greater detail.

ator such as graphite or.beryllium.' As shown in Fig the reactor maycomprise uranium rods 27 enclosed in,

rectangular graphite blocks 28 preferably of square erosssection. Theseblocks with the uranium are referred to as live graphite. The. blocks28in one particular structure,

capable of maintaininga self-sustaining neutronic rcaction, are 4%inches by 4% inches by 50 inches, having...

been planed by woodworking machinery to have smooth rectangular sides.

fit to provide good heat conduction between the uranium and thegraphite. l have also shown in Fig. 3 therelation The reactor 15comprises uranium and a solid-modem. 1

The graphite blocks are drilled and Y reamed andtheuranium rodsslippedtherein wit-ha snugw piped shape and is built, as laterdescribed, in. layers,

one supported on another. As a necessary step in following myinvention,,.I build the: complete reactor. 15 to 6 sign the reactor '15that a greater or active portiondll other or inactive portion 31; isconsiderably l'essjthan critical sizer The-total volume ofthe active.anclfin;

active portions 30and 31 is greater than critical size but less thantwice criticals'ize, The boundary betweenthe activeand inactive portionsis defined by,a I

movable control plate 32 extending through the proje ction 17 toward andpreferably n1ovable.into the projec: tion 192 I also provide'a secondmovable control plate extends frorn the opposite direction to that .ofcontrol plate 32 and through the projection 19 toward andlpr eferablymovable into the projection l 17.

esgnt lrp at s l drfit e w asimiu rs ectom .of other material havinghigh ncut fonfabsorptionechar r a size exceeding that corresponding tocritical size. The, plan crosssection of the reactor 15. may be squareasl shown in the'drawings, the height of the reactor being greater thanthe dimension of theothersides. I so de thereof (Fig.5) is just lcssthancritical .size, and an- 34 in a position above thecontrol p1a te32such thagit.

I acteristics. The plates have a minimum thickness of Arinch althoughthey may be made about one inch in thickness for purposes of rigidity.Alternatively thin cadmium plates may be backed with a steel sheet orother rigid member for purposes of support. The plates 32 and 34preferably can be extended at least two-thirds across the width of thereactor. The plates may be artificially cooled such as by provision ofwater circulating channels in a backing plate. Such cooling isunnecessary in .a reactor operating at low neutron density levels, suchas when the reactor is cooled by natural heat radiation therefrom.

,Referringparticularly to Figs. 4 and 5, the reactor comprising thegraphite blocks 28 filled with the uranium rods 27 are arranged inlayers, the spacing of the rods from one another being dependent uponthe particular cross-section size chosen for the graphite blocks. theparticular structure referred to above that I describe to explain theadvantages accruing from my invention, I provide a reactor incorporatingin the active portion thereof, 576 uranium rods and graphite blocks,each rod having a total length of 16.75.feet. The uranium rods may, ofcourse, be of several sections laid end to end within the graphiteblocks, the blocks having a density of 1.65 gm./cm. I arrange theseblocks in 24 layers of. 24 graphite blocks in each layer resulting in acubical active portion 30 slightly less than critical size, measuringapproximately 16.75 feet on a side. Surrnounting the active portion 30,I provide a total of 312 uranium rods enclosed in similar 8% inchrectangular graphite blocks or a total of 13 rows of 24 each. The totalheight of the structure thus built up, including space between theactive portion 30 and inactive portion 31 for the control plate 32, andthe space in the inactive portion 31 for the control plate 34, isapproximately 26 feet.

The number of rows of uranium filled graphite blocks between the controlplates 32 and 34 may vary depend- 7 ing upon the degree of controldesired for a given movement of the'control plates. For example, if thecontrol plates are locked together such as by an external yoke and movedtogether such that while one is moving inwardly the other is movingoutwardly of the reactor, a given movement will provide a change in sizeof active portion dependent upon the number of uranium rods and graphiteblocks between the control plates.

For uranium rods of substantially pure uranium hav- I ing a boroncontent of less than 0.5 part per million and having a diameter of 3.4centimeters spaced 8% inches apart in a square lattice by high qualitygraphite, the reproduction factor K is approximately 1.074. The criticalsize of a neutronic reactor may be computed for a cubical structure withthis value of K to determine the approximate desired size for the activeportion 30 of the reactor using the above formula relevant to arectangular parallelepiped. A cubical reactor for this value of K is17.35 feet on a side. Since, in accordance with my invention, it isnecessary to provide the portion of the reactor under the control plate32 smaller than critical size, the active portion 30 is made cubicalsuch as 16.75 feet on a side. For this structure the reproduction ratioR is approximately 0.995 so that it is impossible to initiate aself-sustaining neutronic reaction in the active portion alone. However,in accordance with my invention, the reaction may be initiated bysupplying neutrons to the active portion 30 from the inactive portion 31of the reactor by effectively adding part of the inactive portion to theactive portion. A further advantage of my invention resides in the factthat I provide compensation for reduction in the reproduction factor ofthe reactor during the operating life thereof. For example, if it beassumed that a one percent reduction in the reproduction factor mayoccur during prolonged operation, the size of the inactive portion maybe predetermined so that upon any reduction in the reproduction factordur ing by relative outward movement ofthe control plates,

thereby effectively increasingthe active portion 30 while decreasing theinactive portion 31.

Assuming the same lattice structure is used for the in-, I activeportion of the reactor, and a possible reduction of one percent in the Kfactor during operation, the height of the inactive portion superimposedupon the active portion may be predetermined from the above formula fora rectangular parallelepiped structure. Under such conditions the heightof the inactive portion is 9 feet, comprising 13 rows stacked verticallyover the active portion for a total of 312 live graphite blocks.

The reactor 15 is preferably completely surrounded by the graphitereflector 16 for the purpose of conserving neutrons for the neutronicreaction. Thus neutrons escaping from the reactor enter the graphitereflector, are slowed by impact with the graphite atoms, the probability of their returning to the reactor being greater than in theabsence of the graphite reflector due to random changes of direction onimpact. The value of the reproduction factor used in computing thevolume of the active portion 30 of the reactor 15 is based on the use ofa graphite reflector 20 inches in thickness completely surrounding thereactor. The volue of the cubical active portion 30 is computed on thebasis of equal neutron loss from allsurfaces of the cube.

The actual construction of the reactor 15 and reflector 16 is started byplacing on the foundation 10 within the vault space 14 several layers ofplain graphite blocks, referred to as dead graphite, to form the portionof the reflector underlying the reactor. This portion is madeapproximately 25 inches in thickness corresponding to 3 rows of solid 8%inch square cross-section graphite graphite are closely packed withoutcement to eliminate.

air spaces therebetween as far as possible. Because of the block type ofconstruction of both the reactor and reflector the structureincorporating these parts is sometimes termed a pile. The constructionis continued until in approximately the fifteenth and sixteenth layer,four graphite blocks 35 (Fig. 3) having their corners cut away are laidto provide the channel 23 extending to the horizontal center of thereactor. The ionization chamber 24. 1s then installed in the deadgraphite reflector with the shielded conduit 26 enclosing the wire line25 extending therefrom to the outside of the structure. The chamber 24is preferably installed opposite the fifteenth and sixteenth layer ofthe live graphite. on the side opposite the channel 23. The use of theionization chamber will be described later in greater detail. g

The regular construction is then continued further until 24 layers oflive graphite have been built up to form the active portion 30 of thereactor.

During the construction of the active portion 30, the front and rearwalls 18 and 20 are built progressively with the assembly of the liveanddead graphite to a height corresponding to 21 rows of live graphitewhereupon the dead graphite is extended to the outer surface of the endwalls to form the projections 17 and 19 of 24th layer of active graphitesupports at the ends'thereof J several blocks 36 of dead graphite about2 inches in heightto form a space 38 for the control plate 32. The

layers-of graphite subsequently .addedover the control I plate aresufliciently rigid to define the space 38 without sass-ass material-sagof-the live graphite blocks into the space 38.

It -is exceedingly important that prior,to continuing the assemblyof thelive graphite to form the inactive migration of neutronsfrom theupperportion 31, subsequently'constructed, into the lower portion30;'therebymaintaining the reproduction ratio of the reactor below unity untilnormal operation is desired.

Following placement of the blocks 36' of dead graphite and introductionof the control plate 32 within thelateral of the reactor between-thecontrolplates 32 and 34. i In the structure described, 6 layers-of livegraphite are assembled,:whereupon-space 40 -is left for the controlplate 34' ina mannersimilar to that used to provide the The presenceof-the -plate 32 in that position substantiallyprevents space 38- by useof additionaldead graphite blocks 42 resting on the thirtieth layeroflive graphite. Control platei34is then inserted fullyin thespace 40 tolimit the contribution of neutrons to theactiveportion 30 uponi-addingthe-seven-remaining layers -of the reactor. The side walls 18= and-=arecontinued and the upper portion :of thereflectoradded as deadgraphite, the top,

of the structure-being completed by the addition of the concretebricks22.

As the. layers-of active graphite are added over the space 38.;enclosingtheplate 32 a point is reached at which,-,without the inserted plate 32,the partially completed. reactor would produce a self-sustainingneutronic reaction. Such a point is shown in Fig. 5 as defined by aplane 44. That portion of the reactor below the plane and including thecontrol plate 32 is physically greater than icritical size whereas thatportion abovethe plane 44nislless-than critical'sizep The entirereactor-may therefore-be-consideredas of two unequal portions eachhaving a controlplate extending therein fromopposite Sid6$wuConsequently,efiectivecritical size at which a 1 selfsustainingwreaction is i justmaintained is reached by withdrawingdthe rcontrol plates in -the manner described beIQW. I. i V Ini-thestructurejust described, control ofthe reactor is effectednbysincreasing-and thendecreasing theefi ective structural volume occupied by the plates.Consequently, a

the. plates 3zaand=34 neednot be continuous butmay be in theaform ofparallel rods of. circular or other sh aped crosssection, supported sideby side=in planes positioned similarlyltoithezr plates 32 and 34, apluralityofsuch rods replacingmach ofnthe plates 32 and 34:

Referring to Fig..-7,iishowing a modified form of a structures made. inaccordance with my invention" and whereinaspartsrpreviously describedare similarlyreferenced, ;.:theioontrol-:plates are each replaced by aplurality of ;rods;.-; and 52,"itthe.;rods 5Q replacing the controlplate; 32;- andr;;the;;rods ,52 replacingthe control plate 34 preyiouslydes 'bed Thesearods, are likewise of cad-,

miupr or other material having high neutron absorption,

characteristics ,and, are supported within graphite blocks 54ari 56 Aslbestshown in Figs. 7 and -8,.the drilled aphit h pqks. 54. d56 a e peferably p s ion d t a verse ly pf the active graphite blocks. 28 whichsupport, theturanium rods 27,"t he remaining portion pf the strupturebeing as previously described.

Withthe structure shown in Figs. 7 and 8, the control 1 rods 50 are tiedtogether and moved as a unit ,withthe I control rods 52 similarly*tied.together and moved in opposite directions as a unit,thereby eife'ctivelyvarying the efiective volume of the reactor within limits above andbelow that corresponding to criticalsizeh i I have plotted as anillustrative curve 60in Fig.9, the neutron density as ordinates from andalong a cen-j tral axis OO in the active portion of the reactor. I i

have also plotted similar curves, representative of neutron density inthe inactive portion of the reactor, the curve 62".. being for theneutron density between. the control plates 32 and 34, and the curve 64being indicative of neutron density for that portion of the reactorabove the control plate 34. It will be noted that the neutron density,as represented by these curves, decreases gradually from the approximatecenter of the respective portions .of the reactor for which the curvesare drawn anddecreases rapidly adjacent the control plates 32 and 34.Thus, it will be noted that the neutron density immediately adjacent thecontrol plates 32 and 34 decreases rapidly to a value that is small incomparison with the maximum neutron densities adjacent the center of therespective reactor portions. This action, previously mentioned; is anatural function of the neutronic reaction andis due. to the fact that.neutrons absorbed by the control-plates are inefiectivein producingadditional neutrons by fission; whereas in the absence of the plates,these neutrons J i would. produce :fissionand a consequent increase in*neutrondensity. The control plates are therefore efiective :over-agreater volume of; the reactor than actually occupied by the .plates.-It is for this reason that adjacentcontrol .rods, such as shown in Figs.7 and 8, may-be used in place. 10f each 'of the control platespreviously described. The curves 60, 62, and 64' shown in Fig. 9 arerepresentative of one condition of operation for a given posi-- tioningof the control plates 32 and 34. For the condi-- tions assumed,.and theoverlapping positioning of the control plates as shown, the neutroncontribution of theportion above plate 32' butnot cut off from theactive portion .30 by. the.- pl ate: 32 .to the active portion 30' of.the reactor is suflic'ient to maintain a selfrsustaining'neutronicreaction in the reactor. Thus the-volume of thereactor effective inmaintaining the reaction is equal to or greater than that correspondingto critical size. L If this effective volume is greater than criticalsize, the neutronic reacw tion-will increase, exponentiallytlwith time:and it is *neces sary to prevent such anexponentialsrise by-additionaluoverlapping, or. by so, moving the control plates 32 and-- 34 in unisonthatv a smaller! portion of the inactivepor tion 31 of the, reactor iseffective insupplying neutrons to the active portion 30; This lattermethod is preferred when only small ranges in reproduction ratio aredesired for operating control purposes.

Let is be assumed that .it is desired to increase the neutron densitywithin the reactor over the condition shown in Fig. 9. The reaction maybe increased in density by moving the control. plate simultaneously tothe right so that the controlplate 32 is slightly withdrawn from thereactorand the control plate 34 is insertedpinto the reactor, by anequivalent amount. This motion inn. creases the neutron contribution tothe active portion' 30 from the portion between plates. 32 and 34. Suchmotion r of the control plates provides. a fine degree of controlneutron density. For comparison purposes, however, Fig. is a curveillustrative of neutron density for nonoverlapping positioning of thecontrol plates 32 'an d"34. It would be exceedingly dangerous towithdraw both plates to the extent shown in Fig. 10 if a neutronicselfsustaining reaction occurred with the plates as shown in Fig. 9 andthis example is given merely for illustrative purposes.

Referring to Fig. I 10, the plates 32 and 34 are shown withdrawn to suchan extent that there is direct contribution of neutrons from theinactive portion to the active portion along the axis of the reactor. Itshould be noted that the curve decreases somewhat in the region beyondthe end of the control plate 32 due to absorption thereby of thoseneutrons that would otherwise produce fission and a consequent increasein density over this region. Similarly, the curve 66 shows an increasein neutron density betweenthe control plates 32 and 34 due to fission inthis region and a second decrease adjacent the control plate 34,increasing slightly inthe volume of the reactor above the control plate34. The adjustment of the control plates as shown in Fig. 10 will bemore criticah: Thus, a given outward movement of the control plates willproduce a greater increase in neutron density for the position shown inFig. 10 than for that in Fig. 9, because for such given movement agreater number of neutrons are eflfective in contributing to theneutronic reaction.

The curves shown in Figs. 9 and 10 are representative of neutron densityalong the axis OO as previously indicated; The neutron density along anaxis transverse to the axis OO will likewise decrease fiom the centertoward the sides of the reactor. Consequently, a given movement of thecontrol plates will be more elfective in reaching or exceeding criticalsize conditions when the overlap of these plates is centered about theaxis 0-O than when this overlap is adjacent one or the other edge of thereactor. I I 1 As indicated above, it is not yet accurately known towhat degree the reproduction factor will increase or decrease withprolonged use of the reactor. The weight of experimental evidence seemsto indicate a variation in the reproduction factor wherein an initialdecrease in the factor is followed by an increase that is in turnfollowed by a second decrease. In the initial stages of reactoroperation it is'expected that the formation of radioactive fissionproducts having large neutron capture characteristics may occur at agreater rate than the formation of plutonium. This would produce areduction in the. reproduction factor. However, measurements'made .onsuch radioactive fission products indicate-that they are'converted byradioactive decay-processes or by neutron absorption to elements orisotopes having smaller neutron capture characteristics. Consequently,the effect of the fission products in increasing neutron absorption willtend to become stabilized and an equilibrium reached between formationand transmutation. However, the

formation of plutonium continues with reactor operation and it isexpected that the increase of fissionable material by the formation ofplutonium-will cause an increase in thereproduction factor after theinitial stages of operation,-thereby overcoming the initial loss due toneutron absorption by the formed fission products. However, in the laterstages of operation the exhaustion of U by fission thereof may not becompensated-fully -by the formation of plutonium, the plutonium is usedup by; fission, and the reproduction factor may again. decrease. It istherefore desirable to provide a reactorin which compensation for thisvariation in the reproduction factor may be made. Thus in theinitialstates of operation during which the reproduction factor inay.decrease the control plates may be moved outwardly of the reactor suchas :to. add tothe etfectivesize,v followed by an inward movementdecreasing the effective size and subsequent outward movement toovercome deficiency in the fissionable material of the reactor.Thiselfect will-be 14 very slow, however, and unless pronounced changes',are' encountered, movement of the control plates-togetherih the samedirection may be suflicient. However, -'for greater changes in thereproduction factor,'a shift in the overlapping relationship of theplates may be'necessary. As indicated above, any-increase in neutrondensity after effective critical size of the reactor is reached, such asby initial adjustment of the control Plates, will be effective inproducing an exponential rise in the rate of neutronic reaction. Such arise must be terminated to maintain the neutronic reaction within theheat dissipating capacity of the reactor and its surrounding reflectorand concrete shield. Thus, following initiation of a self sustainingneutronic reaction, the reaction may be sta bilized about an averageneutron density value within the heat dissipating capacity of the systemby increasing the relative overlapping condition of the control platesor by moving the control plates as a unit so that'the overlap thereof isin a lower density region of the reactor until the condition of lessthan unity reproduction ratio is obtained and the neutron density decaysto the newly desired lower level. Following attainment of this desireddensity, thecontrol plates may be moved to a slightly higher neutrondensity position to raise the reproduction ratio above unity at whichthe neutronic reactiontends to increase, whereupon the separation orposition of the control plates may be varied above and below thatcorresponding to critical size with a reproduction ratio of unity.

It should be appreciated that the action of the control plates inlimiting and controlling the neutronic reaction is not due to variationin neutron absorption with variation in position of the plates for agiven power. Thus, after normal operation is initiated and for thefinest degree of control, the plates are moved simultaneously and in thesame direction, so that a decrease in volume of one plate within thereactor is compensated by an increase in volume of the other plateinside the reactor. While the neutron absorptionyby each plateindividuah 1y varies somewhat in accordance with the volume thereofwithin the reactor, the total neutron absorption by both plates or bythe two sets of control rods is substantially constant for differentpositions thereof, and almost exactly constant for the small distancesthe plates move to exert normal control. a

The materials of which the control plates are made, such as cadmium,have a larger neutron absorbing action for slow neutrons than for fastneutrons. Some few fast neutrons, incidenton the control plates from theactive portion 30 of the reactor may penetrate the plates and producefission by being slowed to thermal'cnergy by the graphite in theinactive portion of the reactor. The proportion of penetrating. neutronswith respect to,ab,- sorbed neutrons is relatively small however, andthe number of the neutrons produced through fission by the penetratingneutrons which repenetrate the control plates and return to the activeportion of the reactor is still smaller. Consequently, the controlplates act as barriers limiting the effective critical size of thereactor'devel'oping the neutronic reaction. In addition to theabovementioned eflect, the action of the control plates is effectivein'decreasing the p'robabilityof fission immediately adjacent thesurfaces of the control plates. .Such action may be termed a neutronsink action that reduces the number of neutrons having sulficient energyto penetrate the control plate, thereby reducing fission in each portionof the reactor by neutrons previously produced in the other portion. 1

Thereaction may be controlled by manual movement of the control platesalthough movement of the control plates in accordance with the neutrondensity within the reactor maybe desired. Reference is made. to Fig. '11that shows diagrammatically one form ofautomatic'control and safetycircuit that may bev used for regulating the neutron density within. thereactor. .The ionization Each of the magnetic clutches is connected inparallel and to a power line 140 through emergency break switches 142.Upon opening one or more of the break switches 142, the magneticclutches are de-energized, allowing the control plates to be drawninwardly by weights 144 and 146 attached to the racks 114 and 126),respectively. Such a safety measure is very desirable in the event ofpower failure that might leave the control plates-in such a positionthat the neutron density would continue to rise indefinitely.

The motor 136 is capable of being energized from power line 148 througha switch lever 150 thrown manually into engagement with either motorcontact 152 or 154 depending upon the desired direction of rotation ashereinafter appears.

It will be apparent that the motor 94 being mounted in a fixed positionis capable of moving the rack 114 to the right or left together with theplatform 138 on which the motor 136 ispositioned. The pinion 122 beingdriven through worm 132 remains stationary so that when the motor 136 isunenergized the rack 120 moves with the rack 114. Thus, as the controlplate 32 is moved inwardly or outwardly of the reactor,the control plate34 is moved outwardly and inwardly, respectively, to the same extent.

Having described a circuit for controlling the position of the controlplates in the reactor, I will now describe its operation, consideringthe condition obtaining When the plates are fully inserted within thereactor such as following construction of the reactor as previouslydescribed.

Since the active portion of the reactor below the controlplate 32 isconstructed to be less than critical size and the inactive portion 31 isineliective in materially adding neutrons because of the fully insertedconof the is made. The movement of the control plate 34 inwardly iscontinued until the non-linear rise in neutron density is overcome. i

In the absence of aself-sustaining neutronic reaction withinthe reactor,the armature 86 is normally in contact with the motor control contact92. The sliding contact 74 on the resistor 76 is then adjusted toprovide sufficient attraction ofthe armature 86 by the relay coil82uponreinitiation of the neutronic reaction to a density somewhat lessthan that previously obtained by outward movement of the control plate34. Power mains 102 are thenenergized to drive the motor 94 in acounterclockwise direction to move; the plate 32 in an outwardlydirection. Since the platform 138 carryingmotor 136 is mounted rigidlywith the, rack 114 and the worm 132 prevents rotation of thepinion 122,movement of the rack114 will produce corresponding movement of the rackand conseguent inward motion ofthe control plate 34. Itwillthus beapparent that as the plate 32 is moved outwardly, the plate 34 is movedinwardly. However, notwithstanding substantially uniform neutronabsorption by the control plates under such movement, the control plate32 being in a lower region of the reactor will tend to increase theeifective size of the reactor at a greater rate than the plate 34produces an etfective decrease in size, and with outward movement of theplate 32 and equal inward movement of the plate 34, a position isreached at which the neutron density within the reactor increasesexponentially. The previously adjusted ionization chamber circuit willthen provide suflicient current to the relay coil 82 to drawthe armature86 into engagement with the motor control contact 90, thereby reversingthe motor 94, with consequent reversal of the motion of the controlplates 32 and 34, to dee on chamber asse ses 17 crease the neutrondensity within the reactor. The con trol plates will then continue tohunt between positions on either side of a condition corresponding tothe critical size of the reactor causing first an increase and then adecrease in neutron density within the reactor. Any well- 'knownanti-hunting device may be used, as well known templates the potentialutilization of the entire active portion of the neutronic reactor. Withdecrease in K in operation of the reactor, the plates 32 and 34 aremoved simultaneously to the right (Fig. 5) untilthe whole volume betweenplates 32 and 34 becomes part of the active portion of the reactor. Thevolume above; the

plate 34 remains substantially inactive during this time.

Then when additional K is required, and as it is demanded, the plates 32and 34 are moved apart to utilize the uranium above the plate 34.Assuming that K continues to fall, ultimately the plates 32 and 34 willbe spaced apart the width of the active portion and the full amount ofuranium originally provided Will be active. It should be distinctlyunderstood that the control by movement of the control plates cannot belikened to a throttle control. The rate at which the reaction occurs isnot dependent upon the volume of the reactor effective in supplyingneutrons to the reaction, but rather upon the neutron density attainedafter exceeding an effective volume corresponding to critical size andbefore decrease to this size. For example, upon increasing the volumebeyond that corresponding to effective critical size by moving thecontrol plates, the neutron density will continue to increaseexponentially with time. therefore elfected by varying the total volumeof the reactor effective in developing the neutronic reaction bymovement of the control plates.

An important element in the control of the reactor is the fact that notall of the fast neutrons originating in the fission process are emittedimmediately. About one percent of the fast neutrons are delayedneutrons. These delayed fast neutrons appear from .01 second to severalminutes after the fission has occurred. Half of these neutrons areemitted Within six seconds and .9 within 45 seconds. The mean time ofdelayed emission is about 5 seconds. The cycle of neutron emission,migration through the moderator, slowing to thermal energy, and

I fission capture iscompleted by 99 percent of the neutrons in about.0015 second, but if the reactor is near the bal-.

anced condition the extra 1 percent may make allthe difference betweenan increase or a decrease in the neutronic activity. The fact that thelast neutron in the cycle is I held back, as it were, imparts aslowness, of response to the reactor that would not be present if theneutrons were all emitted instantaneously.

For cases in which the reproduction ratio R differs from unity by lessthan 1 percent, the ratio of rise is given by n=ne" where a (R--1) T Inthis formula or. is the fraction of the neutrons that are delayed,m=.01, T is the mean lifetime of the delayed neutrons=5 second.

As an example, assume R becomes 1.001 as a result of increasing theactive volume of the reactor by mov- Control is increased severalpercent, so that the one percent delayed I 1's ing' the plate 32outwardly and the plate 34 inwardly of the reactor. Then that is, n/n=2.72 in 45 seconds. This doubling occurs about every 30 seconds andcontinues indefinitely.

If R were made exactly 1.01, a more detailed theory shows that theneutron density would be tripled each second. However, if thereproduction ratio R is suddenly neutrons are unimportant as comparedwith R1, the neutron density increases at a much more rapid rate asgiven approximately by R/ l where l is .0015 second, the normal time tocomplete a cycle. Thus, if R were to be made 1.04, the neutron densitywould increase in 1.5 seconds by a factor of approximately 10 over itsoriginal level. However, if R were 1.02 or 1.03, the factor by which theneutron density would be multiplied each second would be 1100 and700,000, respectively. It is thus apparent that too high a reproductionratio in a practical system leads to the necessity of providing safetymeasures which positivelylimit all danger of exceeding a permissiblerate of neutron density increase. A11 exceedingly dangerous conditioncould exist if by accident the effective volume of the reactor weresuddenly increased considerably beyond that corresponding to criticalsize as the time required for inserting the control plates might be toolong to prevent destruction of the system. As the same eventual neutrondensity can be obtained with a reproduction ratio only slightly overinterest of safety.

From the foregoing description of a structure embody ing my inventionand my method of controlling a neutronic reaction, it will be apparentthat the-reactor may be built to a physical size exceeding critical sizeto. allow for any decrease in the reproduction factor of the reactorduring continued operation thereof. During the operation of a neutronicreactor particularly at high neutron densities radioactive elements ofexceedingly high capture cross section may be formed in the uranium asan intermediate element in the decay chains of fission fragments andthis formation will lower the value of the reproduction factor for thesystem. Radioactive xenon 135 is an example of such an intermediateelement, this product having a half life of about 9 hours and beingformed mostly from radioactive iodine which has a half life of about 6.6hours and decays to barium. My. invention provides reserve reproductionfactor to compensate for such poisoning. Notwithstanding the larger thancritical size construction, the volume of the reactor effective indeveloping the reaction may be limited by separating the reactor intounequal parts by neutron absorbing members without danger of exceedingthe heat dissipating ca-' pacity of the reactor and without danger ofproviding a high reproduction ratio that might cause such a rapid risein neutron density as to be uncontrollable. 4 Simultaneous movement ofthe control plates inthe same direction provides mutually opposingeffects so that a small movement of one plate is counterbalanced by themovement of the other plate. Accordingly, the increase or decrease fromcritical size conditions is retarded with devices in the drawings,temperature responsive controlsmay be used to interrupt the magneticclutch circuits to draw the plates within the reactor. Auxiliaryionization chambers may be installed adjacent the reactor to similarlydraw the plates inwardly under high eXCQSS the graphite or othermoderator, such as. beryllium. The uranium may also be enriched by otherfissionable ma terials such as by 94 incorporated tlgerewith.

Corresponding to a maximum value of the reproduc:

tion factor of K=1.092 for pure natural uranium metal rods in puregraphite, the following maximum values of K for other pure materials andshapes is given here for illustrative purposes:

Natural U rods in graphite K 2 1 Natural U02 rods in graphite K: 1.058Natural U308 rods in graphite K: 1.047 Natural U spheres in graphite K:1.10 Natural U02 spheres in graphite K: 1.066 Natural Usos spheres ingraphite K: 1.055

The reactor may be utilized as a heat energy source and as a highintensity source for neutrons and gamma rays. While'I have not shown anycooling means other than cooling by natural radiation from thestructure, the heat may be removed by a circulating fiuid such as watercarried in conduits embedded in the reactor, in the neutron reflector orin the shield. The channel 23 extending to the centerof the reactorallows escape from the reactor of a collimated beam of fast neutrons andgamma rays. The neutrons may be used for determining characteristics ofmaterials under fast or slow neu tron bombardment and the gamma raysfortaking X-ray photographs of heavy castings or similar equipment wherevery penetrating rays of high density are required. In addition, thechannel 23 may be packed with any material desired and the scatteringeffect of. the material or its neutron absorptioncharacteristicsdetermined, for example. Further isotopes of ortransmutations to, variouselements may be produced in large quantitiesby packing these elements into the channel 23 and expos; ing them to theaction of the neutrons developed in the reactor. As an example ofisotope production, followed by transmutation, U may be produced from Thin the channel 23 in accordance with the product of slow neutron densityin the channel and time of exposure. Thorium 233 then decays to formprotoactinium 2 33 and thence to uranium233 which is valuable as afission able material similar in its, action to U and 94 My neutronicreactor and method of control is of particular advantage in irradiating,within the channel 23, materials highly absorbent toneutrons such assamarium and gadolinium. As indicated above, even small quantities ofsuch materials may stop the neutronic reaction in the reactors proposedheretofore, whereas in my reactor the reproduction ratio may temporarilybe increased greatly to allow for added neutron loss by partiallywithdrawing the plates providing a smaller mutual overlap thereof or bymoving the plates as a unit with consequent greater contribution of theinactive portion of the reactor to the active portion. Upon removal ofthe irradiated materials from the channel 23, the control plates may bereinserted or moved to their original position. Thus, adequate controlis provided before, during, and subsequent to insertion of highlyneutron absorbent materials with hes ann .3-

- In addition to the usual industrial hazards during the qpatation f'theapparatus described, operating personnel must, be protected from injuryby gamma rays and neutrons generated in the reactor. This is the majorpurpose of the 5 foot. thickness concrete shield surrounding the reactorand reflector. Inasmuch as the concrete contains water: ofcrystallization and may contain water retaining materials, and isrelatively dense, it serves as an effective shield for neutrons as wellas gamma rays. Sufficient shielding should be provided to reduce theradiation from the structure to at least 0.10 roentgen per 8 hour dayper person at the closest point of approach. This exposure is consideredto be the maximum safe radiation permissible to which an individual maybe subjected over the whole body.

While the theory of the nuclear chain fission mechanism in uranium setforth herein is based on the best U pr es 1er tly lgnown experimentalevidence, I do not wish to bebound thereby, as additional experimentaldata later discovered may modify the theory disclosed. Any suchmodification of theory, however, will in no way affect the results to beobtained in the practice of the invention herein described and claimed.

I claim I 1. In a neutronic reactor having an active portion comprisinga thermal neutron fissionable material and having a neutron reproductionratio at least equal to unity,

the improved control apparatus comprising a plurality of absorbermembers extending into the active portion at different distances fromthe center thereof, and motive means, coupled in common to said absorbermembers in a manner. to simultaneously withdraw at least one of saidabsoi ber members and to. insert at least one other of said absorbermembers to eifect fine control of the neutron reproduction ratio.

2. The apparatus of claim 1 wherein the total amount ofabsorber materialpresent in the reactor remains con stant with energization of saidmotive means.

3. A method of controlling the neutron reproduction ratio of a neutronicreactor comprising simultaneously withdrawing and inserting respectiveneutron absorber members at respectively different distances from thecenter of the reactor.

'4. The method of claim 3 wherein the total quantity of absorber presentin the reactor is maintained constant.

References Cited in the file of this patent UNITED STATES PATENTS OTHERREFERENCES Kelly et al.: Physical Review 73, 1135-9 (1948).

1. IN A NEUTRONIC REACTOR HAVING AN ACTIVE PORTION COMPRISING A THERMALNEUTROM FISSIONABLE MATERIAL AND HAVING A NEUTRON REPRODUCTION RATION ATLEAST EQUAL TO UNITY, THE IMPROVED CONTROL APPARATUS COMPRISING APLURALITY OF ABSORBER MEMBERS EXTENDING INTO THE ACTIVE PORTION ATDIFFERENT DISTANCES FROM THE CENTER THEREOF, AND MOTIVE MEANS COUPLED INCOMMON TO SAID ABSORBER MEMBERS IN A MANNER TO SIMULTANEOUSLY WITHDRAWAT LEAST ONE OF SAID ABSORBER MEMBERS AND TO INSERT AT LEAST ONE OTHEROF SAID ABSORBER MEMBERS TO EFFECT FINE CONTROL OF THE NEUTRONREPRODUCTION RATIO.